The present invention relates to a method and device in a size press in which a nip is formed by a pair of press rolls. A paper or board web is passed through the nip. A first press roll is permanently mounted, or fixed, by means of bearings on a frame of the size press, while a second press roll is mounted on the frame of the size press by means of additional bearings. The second press roll is displaceable by means of loading arms or equivalent loading means. The press rolls are coated by means of coating devices which spread films of a coating agent onto the faces of the rolls. The coating devices are mounted on applicator beams arranged transverse to the machine direction. The applicator beams are supported pivotally on the frame of the size press or on the loading arms of the displaceable roll and are provided with pivot cylinders. By means of the pivot cylinders, the applicator beams can be opened and closed in relation to corresponding press rolls against which the coating devices operate. In addition, the applicator beams are provided with catches arranged to be supported directly or indirectly on the bearings of corresponding rolls when the applicator beams are being closed by means of the pivot cylinders.
In the prior art, it is a typical construction of modern size presses that the size press comprises a pair of rolls which form a nip through which a paper or board web is passed. A coating agent, such as a size or pigment coating, is applied as a film onto the faces of the rolls by means of application devices. The coating agent is transferred onto the web to be coated in the roll nip. Normally, one roll in the pair of rolls is permanently mounted, i.e. fixed, on the frame of the size press by means of bearings, whereas the other roll is mounted on the frame displaceably, e.g., by means of pivot arms. The pivot arms permit the nip to be opened and the press roll to be loaded against one another so as to produce a desired nip pressure.
A significant disadvantage of the size-press construction described above is manifested in particular in connection with the replacement of the rolls. In particular, when the fixed roll is replaced, the new roll is not always positioned optimally on the frame of the size press, rather it may remain slightly inclined. When the nip is closed, the displaceable roll is positioned in a way corresponding to the fixed roll to produce a uniform nip pressure. Thus, the displaceable roll is also positioned in an inclined manner in a way corresponding to the fixed roll.
The application devices of the rolls are typically mounted on an applicator beam arranged transverse to the machine direction. The applicator beam is linked pivotally on the frame of the size press or on the loading arm of the displaceable roll. The applicator beam is usually provided with pivot cylinders, by whose means the beam and the coating device mounted on the beam are "closed" against the corresponding roll, i.e. to form a nip. However, the construction of the applicator beam is very rigid, for which reason it cannot always be positioned by means of its pivot cylinders in a manner corresponding to the positions of the rolls.
An applicator beam is generally provided with mechanical catches, which rest against catch faces formed on the bearing housings of the roll when the beam is closed. If the roll has been positioned in a highly inclined position, upon closing of the beam, a situation may arise in which the catch placed at one side of the beam only reaches contact with the bearing housing of the roll. Therefore, a gap remains between the catch placed at the opposite side and the catch face formed on the bearing housing. Even if the coating member of the coating device could be placed correctly against the roll face, by means of its loading hose, the pivot cylinders of the applicator beam usually produce an error in the nip pressure. This error is increased linearly in the transverse direction of the machine and has a highly detrimental effect on the coating result produced by the application devices on the press rolls.
Hydraulic catches have also been applied in applicator beams of size presses. In their simplest form, they consist of a hydraulic cylinder whose length can be adjusted, i.e., a "jack". By adjusting the length of the hydraulic cylinders, both ends of the applicator beam can be made to rest against the catch faces formed on the bearing housings of the roll. However, the drawbacks related to such a solution are similar to those of the mechanical catches described above, i.e., the regulation is difficult, because the two catches must be adjusted separately. The support forces of both of the catches would have to be adjusted equal in order to prevent the pivot cylinders of the applicator beam from producing an error in the nip pressure. More "advanced" hydraulic catches invariably involve complicated control and regulation circuits, which are also expensive. Also, hydraulic systems always involve an unpredictable risk of leakage.